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= cowboy_websocket(3)

== Name

cowboy_websocket - Websocket

== Description

The module `cowboy_websocket` implements Websocket
as a Ranch protocol. It also defines a callback interface
for handling Websocket connections.

== Callbacks

Websocket handlers must implement the following callback
interface:

[source,erlang]
----
init(Req, State)
    -> {cowboy_websocket, Req, State}
     | {cowboy_websocket, Req, State, Opts}

websocket_init(State)            -> CallResult  %% optional
websocket_handle(InFrame, State) -> CallResult
websocket_info(Info, State)      -> CallResult

terminate(Reason, PartialReq, State) -> ok      %% optional

Req        :: cowboy_req:req()
PartialReq :: map()
State      :: any()
Opts       :: cowboy_websocket:opts()
InFrame    :: ping | pong | {text | binary | ping | pong, binary()}
Info       :: any()

CallResult :: {commands(), State}
            | {commands(), State, hibernate}
            | Deprecated

Deprecated :: {ok, State}
            | {ok, State, hibernate}
            | {reply, OutFrame | [OutFrame], State}
            | {reply, OutFrame | [OutFrame], State, hibernate}
            | {stop, State}
OutFrame   :: cow_ws:frame()                    %% see types below

Reason     :: normal | stop | timeout
            | remote | {remote, cow_ws:close_code(), binary()}
            | {error, badencoding | badframe | closed | atom()}
            | {crash, error | exit | throw, any()}
----

The `init/2` callback is common to all handlers. To upgrade
the connection to Websocket, it must return `cowboy_websocket`
as the first element of the tuple.

Any operation requiring the HTTP request must be done in the
`init/2` function, as the Req object will not be available
after it returns. Websocket sub-protocol selection should
therefore be done in this function.

The optional `websocket_init/1` callback will be called once
the connection has been upgraded to Websocket. It can be used
to perform any required initialization of the handler.

Note that the `init/2` function does not run in the same
process as the Websocket callbacks. Any Websocket-specific
initialization must be done in `websocket_init/1`.

The `websocket_handle/2` callback will be called for every
frame received. The `websocket_info/2` callback will be
called for every Erlang message received.

All three Websocket callbacks may send one or more frames
back to the client, including close frames to terminate
the connection; enable/disable active mode; enable/disable
compression for subsequent frames; or change Websocket options.

The optional `terminate/3` callback will ultimately be called
with the reason for the termination of the connection. This
callback is common to all handlers. Note that Websocket will
not provide the full Req object by default, to save memory.

Cowboy will terminate the process right after closing the
Websocket connection. This means that there is no need to
perform any cleanup in the `terminate/3` callback.

The following terminate reasons are defined for Websocket
connections:

normal::
    The connection was closed normally before establishing a Websocket
    connection. This typically happens if an `ok` tuple is returned
    from the `init/2` callback.

remote::
    The remote endpoint closed the connection without giving any
    further details.

{remote, Code, Payload}::
    The remote endpoint closed the connection with the given
    `Code` and `Payload` as the reason.

stop::
    The handler requested to close the connection, either by returning
    a `stop` tuple or by sending a `close` frame.

timeout::
    The connection has been closed due to inactivity. The timeout
    value can be configured from `init/2`.

{crash, Class, Reason}::
    A crash occurred in the handler. `Class` and `Reason` can be
    used to obtain more information about the crash.

{error, badencoding}::
    A text frame was sent by the client with invalid encoding. All
    text frames must be valid UTF-8.

{error, badframe}::
    A protocol error has been detected.

{error, closed}::
    The socket has been closed brutally without a close frame being
    received first.

{error, Reason}::
    A socket error ocurred.

== Types

=== commands()

[source,erlang]
----
commands() :: [Command]

Command :: {active, boolean()}
         | {deflate, boolean()}
         | {set_options, #{idle_timeout => timeout()}}
         | {shutdown_reason, any()}
         | Frame :: cow_ws:frame()
----

Commands that may be returned from Websocket callbacks.

The following commands are defined:

active::

Whether to disable or enable reading from the socket. This
can be used to apply flow control to a Websocket connection.

deflate::

Whether the subsequent frames should be compressed. Has no
effect on connections that did not negotiate compression.

set_options::

Set Websocket options. Currently only the option `idle_timeout`
may be updated from a Websocket handler.

shutdown_reason::

Change the shutdown reason. The Websocket process will exit
with reason `normal` by default. This command can be used to
exit with reason `{shutdown, ShutdownReason}` under normal
conditions. This command has no effect when the Websocket
process exits abnormally, for example following a crash in a
handler callback.

Frame::

Send the corresponding Websocket frame.

=== cow_ws:frame()

[source,erlang]
----
frame() :: {text, iodata()}
    | {binary, iodata()}
    | ping | {ping, iodata()}
    | pong | {pong, iodata()}
    | close | {close, iodata()} | {close, close_code(), iodata()}

close_code() :: 1000..1003 | 1006..1011 | 3000..4999
----

Websocket frames that can be sent as a response.

Note that there is no need to send pong frames back as
Cowboy does it automatically for you.

=== opts()

[source,erlang]
----
opts() :: #{
    active_n       => pos_integer(),
    compress       => boolean(),
    deflate_opts   => cow_ws:deflate_opts()
    idle_timeout   => timeout(),
    max_frame_size => non_neg_integer() | infinity,
    req_filter     => fun((cowboy_req:req()) -> map()),
    validate_utf8  => boolean()
}
----

Websocket handler options.

This configuration is passed to Cowboy from the `init/2`
function:

[source,erlang]
----
init(Req, State) ->
    Opts = #{compress => true},
    {cowboy_websocket, Req, State, Opts}.
----

The default value is given next to the option name:

active_n (100)::

The number of packets Cowboy will request from the socket at once.
This can be used to tweak the performance of the server. Higher
values reduce the number of times Cowboy need to request more
packets from the port driver at the expense of potentially
higher memory being used.
+
This option does not apply to Websocket over HTTP/2.

compress (false)::

Whether to enable the Websocket frame compression
extension. Frames will only be compressed for the
clients that support this extension.

deflate_opts (#{})::

Configuration for the permessage-deflate Websocket
extension. Allows configuring both the negotiated
options and the zlib compression options. The
defaults optimize the compression at the expense
of some memory and CPU.

idle_timeout (60000)::

Time in milliseconds that Cowboy will keep the
connection open without receiving anything from
the client.
+
This option can be updated at any time using the
`set_options` command.

max_frame_size (infinity)::

Maximum frame size in bytes allowed by this Websocket
handler. Cowboy will close the connection when
a client attempts to send a frame that goes over
this limit. For fragmented frames this applies
to the size of the reconstituted frame.

req_filter::

A function applied to the Req to compact it and
only keep required information. The Req is only
given back in the `terminate/3` callback. By default
it keeps the method, version, URI components and peer
information.

validate_utf8 (true)::

Whether Cowboy should verify that the payload of
`text` and `close` frames is valid UTF-8. This is
required by the protocol specification but in some
cases it may be more interesting to disable it in
order to save resources.
+
Note that `binary` frames do not have this UTF-8
requirement and are what should be used under
normal circumstances if necessary.

== Changelog

* *2.8*: The `active_n` option was added.
* *2.7*: The commands based interface has been documented.
         The old interface is now deprecated.
* *2.7*: The command `shutdown_reason` was introduced.
* *2.7*: The option `validate_utf8` has been added.
* *2.6*: Deflate options can now be configured via `deflate_opts`.
* *2.0*: The Req object is no longer passed to Websocket callbacks.
* *2.0*: The callback `websocket_terminate/3` was removed in favor of `terminate/3`.
* *1.0*: Protocol introduced.

== See also

link:man:cowboy(7)[cowboy(7)],
link:man:cowboy_handler(3)[cowboy_handler(3)],
link:man:cowboy_http(3)[cowboy_http(3)],
link:man:cowboy_http2(3)[cowboy_http2(3)]